A major force driving the technology evolution of fiber-optic transport networks is the desire to lower the cost per transmitted bit. With the rapid growth of capacity demands and limited available optical bandwidth, maximizing transport spectral efficiency is becoming increasingly important for lowering such costs. One focus of transport spectral efficiency is to improve on the capacity limitations of common network infrastructure, such as the fiber-optic cable.
In recent years, various high-order modulation formats with different optical reach capabilities have been proposed for increasing transport spectral efficiency. However, many of these approaches put a severe limitation on the design of optical networks, where a variety of reach demands may be required for different wavelength channels and at different points in time.